Bipolar transistors such as IGBTs (Insulated Gate Bipolar Transistors) are widely used in different applications in the automotive and industrial field. For example, in motor control or drives applications, IGBTs are often used in a power stage for driving a load. Within such applications, the IGBTs may be cyclically switched on and off.
In electronic circuits, especially in power circuits that drive a load, diodes can be employed as free-wheeling elements that may prevent damage to transistors by suppressing voltage spikes or transients which may occur in switching operations.
In operation of bipolar semiconductor device, such as bipolar transistors and diodes, conduction losses occur. These conduction losses are dependent on a voltage across the respective device and current through the device.
Further, a charge carrier plasma with n-type charge carriers and p-type charge carriers is stored in a bipolar semiconductor device when the semiconductor device is in a conducting state, that is, when the semiconductor device is conducting a current. When an operation state of the semiconductor device changes from a conducting state to a non-conducting state these charges have to be removed from the respective device. This process is often referred to as reverse recovery process. During the reverse recovery process, the charge carriers flowing out of the device cause a reverse current. This reverse current multiplied with the voltage across the device during the reverse recovery process equals the power losses resulting from the reverse recovery process. The time integral of these losses equals the energy that is dissipated each time the device changes from the conducting state to the non-conducting state.
For example, in a half bridge circuit, that includes two semiconductor devices connected in series, losses are not only induced in the device that switches off but also in the other device that is going to take over the load current as this device sees typically a very high voltage, this device may e.g. be a freewheeling diode.
In general, at a given current rating and a given voltage blocking capability, devices with a low forward voltage (and low conduction losses) have a higher reverse recovery charge, and vice versa. Usually, the current rating of a device is chosen in accordance with the highest currents occurring in an application in which the device is implemented. A device with a high current rating has a large chip size and has a high reverse recovery charge. When the device operates at currents below the current rating, the device is over-dimensioned so that at low currents relatively high losses resulting from the reverse recovery charge occur.
It is therefore desirable to reduce transistor and diode losses in electronic circuit applications, in particular in power electronics applications.